Oligonucleotide tags have frequently been employed to label and sort polynucleotides in analytical molecular biology, e.g. Brenner et al, Proc. Natl. Acad. Sci., 97: 1665-1670 (2000); Church et al, Science, 240: 185-188 (1988); Shoemaker et al, Nature Genetics, 14: 450-456 (1996); Hardenbol et al, Nature Biotechnology, 21: 673-678 (2003); and the like. The benefits of conducting analytical reactions with such molecular tags include (i) achievement of high degrees of multiplexing so that many analytes can be measured in the same reaction mixture with conservation of rare or expensive reagents, and (ii) ability to design oligonucleotide tags to optimize assay sensitivity, convenience, cost, and multiplexing capability. Inmost approaches, oligonucleotide tags are attached to polynucleotide analytes or probes in separate reactions, after which they are combined for multiplexed reactions, e.g. Church et al (cited above); Shoemaker et al (cited above); Hardenbol et al (cited above); Wallace, U.S. Pat. No. 5,981,176; and the like. Alternatively, unique oligonucleotide tags have also been attached to sets of polynucleotides in the same reaction by first forming a population of conjugates with a much larger set of oligonucleotide tags followed by removing a sample of polynucleotides (small in size relative to the oligonucleotide tag population), e.g. Brenner et al (cited above); Mao et al, International patent publication WO 02/097113. In the former approach, each analyte or probe may be identified in parallel by reading its oligonucleotide tag (whose sequence is known) in a single operation, e.g. by hybridization to a microarray. While such a readout is extremely efficient, the initial cost of synthesizing and separately labeling the analytes or probes is high. In the latter approach, the cost of attaching tags is low; however, the identity of the oligonucleotide tag attached to a given analyte or probe (even though unique) is unknown, so its use is limited to shuttling information about its probe or analyte to a readout platform.
It would be highly useful if a tagging method were available in which probes or analytes could each be uniquely labeled with an oligonucleotide tag in one or a few multiplex reactions employing a few tagging reagents, such that the resulting tags could be readily identified by a simple decoding procedure. Such a method would have the benefits both of the approach of separately attaching oligonucleotide tags (facile identification) and the approach of attaching oligonucleotide tags in multiplex reactions (less expense). Such a tagging method would find applications in many fields of scientific and biomedical research, particularly in genetics and cancer research where it is frequently necessary or desirable to analyze large numbers of polynucleotide analytes in rare or expensive samples.